Modern vehicles are furnished with a plurality of relays, valves and other components which, with regard to their electrical properties, represent ohmic inductive loads. In order to switch these elements, so-called switching output stages are provided, whose transistors are designed in different sizes, depending on the power to be switched. The transistors are connected in inverse feedback voltage (as diodes), a component being connected in the degenerative feedback path which determines the Zener voltage of the transistors. The transistors act as Zener diodes that drain off the interrupting current when the inductive load is switched off.
To increase the switching capacity, the switching output stages can be connected, in principle, in any parallel manner. The admissible switching current is then yielded by the sum of the individual switching currents of the switching output stages connected in parallel. Upon switching off, the breaking energy and the breaking power do not, however, increase corresponding to the expected values, because of the parallel connections. Thus, one does not obtain the sum of the individual breaking energies of the individual output stages that are connected in parallel.
This applies in an especially extreme manner to a parallel connection of switching output stages and output stages having different breaking power or different Zener voltage, which, for example, can also occur in a tolerance-conditioned manner. It is true that, in such circuits, the admissible switching current and the admissible switching power can be increased, but not so for the admissible breaking power and breaking energy. Rather, the admissible breaking energy is only of the order of magnitude of the weakest output stage. Because of this fact, the range of use of the output stages is severely restricted, and, these days, for the switching of ohmic inductive loads, only output stages and output stage transistors of the same power class are connected in parallel.
However, even in the case of the parallel connection of output stages of the same power class, problems may arise that are conditioned upon tolerances, because output stages and output stage components used these days have manufacturing-conditioned tolerances in the Zener voltage of ±1.5 Volt at a Zener voltage specified to be the same, on a monolithically integrated output stage chip.
Integrated output stage components include, at this time, up to 18 individual output stages that are designed for switching different switching currents. It is an object of the present invention, in a breaking procedure, to reduce the load of individual transistors of the individual output stages or individual switching output stages, connected in parallel, and to effect as uniform as possible a current distribution to all transistors of the individual output stages, and thus to achieve that the sum of the breaking energy is essentially equivalent to the sum of the individual breaking energies.